1. Field of the invention
The present invention relates to a method of Secondary Ion Mass Spectrometry (SIMS) analysis, and more particularly to a method of SIMS analysis which enables high sensitivity mass spectrometry of two or more elements of different electro-negativities in the same analysis.
2. Description of the prior art
The SIMS is a method for evaluating specimen construction by obtaining a concentration distribution of an element in a direction toward the depth and/or the surface of a specimen. Such concentration distribution is obtained by irradiating a primary ion beam of O.sub.2.sup.+, Cs.sup.+, Ar.sup.+, N.sub.2.sup.+ or the like toward a surface of a specimen in a highly evacuated vacuum chamber to emit neutral atoms, secondary ions and secondary electrons by a sputtering phenomenon and selecting a specific secondary ion by means of a secondary ion mass spectrometer. It is possible to detect not only ratios but also concentrations of impurity elements or constitutional elements in a specimen by determining the secondary ion current intensity as the sputtering proceeds.
Up to the present, it is usual to use a specific primary ion source selected in accordance with the degree of electro-negativity of an element to be detected, in order to get high detection sensitivity by SIMS. For example, in case an element having a large electro-negativity such as sulfur (S) is detected, Cs.sup.+ which is a positive ion is irradiated as a primary ion to reduce the work function of the element to be detected and to accelerate emission of the negative secondary ion S.sup.-. To the contrary, in case an element having a small electro-negativity such as chromium (Cr) is detected, O.sub.2.sup.+ or O.sup.- which is a negative ion is irradiated as a primary ion to accelerate emission of the positive secondary ion Cr.sup.+.
Accordingly, when concentration distributions of plural elements having much different electro-negativities should be determined, for example, as shown in the attached FIGS. 1(a) and 1(b), when concentration distributions of Cr and S in a specimen produced by forming an S-doped GaAs epitaxial layer 12 on the surface of a Cr-doped GaAs substrate 11 should be determined, it is usual to employ a method as explained below.
First, as shown in FIG. 1(a), the specimen is placed in a highly evacuated vacuum chamber 21 for a secondary ion mass spectrometer and an O.sub.2.sup.+ ion beam 1 is irradiated as a primary ion beam to sputter the surface of the specimen and detect the sputtering Cr.sup.+ 5 emitted as a secondary ion, thereby carrying out a first analysis to obtain Cr concentration distribution toward the depth direction. In this step, an etching hole 13a is formed on the surface of the specimen by the irradiation of the O.sub.2.sup.+ ion beam 1.
By the irradiation of the O.sub.2.sup.+ ion beam 1 as above, it is possible to emit the sputtering Cr.sup.+ 5 with high efficiency, but emission of S having a large electro-negativity as a secondary ion is low. Therefore, a second analysis is carried out, as shown in FIG. 1(b), by irradiating Cs.sup.+ ion beam 2 as a primary ion beam to the surface of the specimen other than the etching hole 13a to sputter the surface of the specimen and detect the sputtering S.sup.- 6 emitted as a secondary ion, thereby to obtain S concentration distribution toward the depth direction. In this step, an etching hole 13b is formed on the surface of the specimen by the irradiation of the Cs.sup.+ ion beam 2.
In the above method, positions of the etching holes 13a and 13b should naturally be differentiated and so the analysis is not carried out in the same region of a specimen. Further, the sputtering rate is different depending upon the primary ion beam source employed. It is therefore essentially necessary to determine depth of each of the etching holes 13a and 13b and on the basis thereof to make normalization of the scale in the depth direction of each of Cr concentration distribution and S concentration distribution toward the depth direction.
In order to solve the above problems, it has been proposed to alternately irradiate O.sub.2.sup.+ ion beam and Cs.sup.+ ion beam or to repeat irradiation of a low energy O.sub.2.sup.+ ion beam, a high energy N.sub.2.sup.+ ion beam and a low energy Cs.sup.+ ion beam in this order toward the same region of the specimen surface.
In any way, up to the present, it has been necessary to carry out the O.sub.2.sup.+ ion beam irradiation and the Cs.sup.+ ion beam irradiation separately in accordance with the degree of electro-negativity of the elements to be detected in the prior art SIMS. Accordingly, it is required to use plural ion guns and so very troublesome and complicated procedures are required for the analysis.